Uncertainty over the future of the South Korean president, Park Geun-hye, is expected to intensify after a Rasputin-like figure at the centre of a mounting political crisis returned to Seoul, a day after thousands of protesters called for Park’s resignation.

The president was bracing herself for further potentially damaging revelations about her relationship with Choi Soon-sil, a close friend of 40 years. Choi has been accused of wielding undue influence at the heart of the South Korean government and of personally profiting from her ties to the country’s first female president.

South Korea: thousands of protesters call for president to resign

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The complex and at times bizarre allegations surrounding Park’s relationship with her alleged éminence grise have gripped South Koreans since media alleged last month that Choi, the daughter of a religious cult leader, had vetted Park’s speeches, gained access to classified documents, and dictated key economic, defence and foreign policies.

Park’s poll ratings have slumped to as low as 17% – the worst since she was elected almost four years ago – amid lurid accounts of the influence Choi has enjoyed in areas ranging from Seoul’s tough line against North Korea to Park’s choice of wardrobe.

Some lawmakers, including those in Park’s ruling Saenuri party, have even voiced concern that she has fallen under the spell of a religious cult, with Choi performing the role of shaman – prompting one opposition MP to claim the country was being ruled by “a terrifying theocracy.”

After staying in Germany for several weeks, Choi, 60, returned to South Korea on a flight from Heathrow in London on Sunday, and is expected to be questioned by prosecutors investigating whether she and senior aides to Park broke the law.

Park, though, is unlikely to be questioned: under South Korea’s constitution, a sitting president cannot be charged with a criminal offence other than insurrection or treason.

“Choi has expressed through her attorney that she will actively respond to prosecutors’ investigation and will testify according to the facts,” Lee Kyung-jae, her lawyer, said on Sunday morning. “She is deeply remorseful that she had caused frustration and despondency among the public.”

Choi is being investigated for using her ties to Park, 64, to solicit $70m (£57m) in funds from conglomerates, including Samsung and Hyundai, for two nonprofit foundations she set up. Choi has denied misappropriating some of the money for personal use.

She is alleged to have secured her daughter a place at a prestigious university by forcing a change to the admission criteria – a claim that has provoked outrage in a country where students endure intense competition to gain places at top schools.

While prosecutors prepare to question Choi, who has never held public office and does not have security clearance, details emerged of her central role in Park’s public and private lives.

She is said to have become a close confidante of Park in the late 1970s, when Park was grieving over the assassination of her father, Park Chung-hee, a dictator who took power in a military coup in 1961 but who was credited with helping turn South Korea into an industrial powerhouse.

Park Chung-hee was killed by his own chief of intelligence at the end of an alcohol-fuelled private dinner.

Choi’s father, Choi Tae-min, befriended Park after her mother, Yuk Young-soo, was killed by a North Korean spy with a bullet intended for her husband. The older Choi claimed that Yuk had appeared to him in a dream and asked him to help her traumatised daughter.

Choi reportedly married six times, had numerous aliases and founded a shady pseudo-Christian cult called the Church of Eternal Life.

Choi Soon-sil and Park grew closer still after the death of Choi’s father in 1994, four years before Park was elected to the national assembly.

Choi’s ex-husband worked as a close aide to Park in the run-up to her successful presidential bid in 2012, and was allegedly with the president on the day of the Sewol ferry tragedy in April 2014, in which 300 schoolchildren, teachers and crew drowned.

At the weekend thousands of protesters accused Park of betraying public trust and of gross mismanagement, and one recent poll showed 70% of voters wanted her to resign or be impeached.

Few observers believe the crisis will lead to her resignation, with little more than a year left of her single five-year term in the Blue House. Opposition parties have demanded an investigation, but have stopped short of calling for her impeachment.

On Tuesday, Park offered the country a brief apology, but insisted Choi’s influence had not extended beyond checking drafts of her speeches soon after she entered the presidential Blue House in early 2013. “Choi is someone who helped me in a difficult time in the past, and I received her help on some of my speeches,” she said.

Files obtained from Choi’s personal computer by the broadcaster JTBC TV, however, suggest Choi edited some of Park’s key speeches and received confidential documents, including files relating to Japan and North Korea.

In an interview with South Korea’s Segye Ilbo newspaper last week, Choi confirmed she had received drafts of Park’s speeches but denied she had influenced government policy or benefitted financially.

Park sought to defuse the controversy by demanding the resignation of 10 of her senior secretaries and promising a cabinet reshuffle, but the move has done little to cool criticism of her conduct.

Some observers believe that Park will ride out calls for her resignation, with opposition parties reluctant to plunge the country into the political uncertainty that would accompany a lengthy impeachment trial.

Victor Cha, a senior adviser and Korea chair at the Centre for Strategic and International Studies in Washington, said Park was unlikely to quit. “Despite calls from the opposition parties for her resignation (which are only likely to grow during the Choi investigation), the more likely outcome is that she may heed demands to leave the Saenuri party, particularly if Saenuri wants to distance itself from Park in the 2017 presidential election year,” Cha wrote.

The Chosun Ilbo, South Korea’s largest newspaper, said the scandal marked “a complete collapse of a president’s ability to run a government”. The conservative newspaper called on Park’s entire secretariat to resign and the appointment of a caretaker prime minister, effectively ending her role in running the country.

“The only way open to her is to pull out of government and put the public good first,” it said. “Many people are ashamed for her. It is time she was, too.”